Moving worm-like creatures

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Hi!
I have a question about some worm-like creatures which appear moving in my 40 gal tank. They are sitting in the gravel and ...

Hi!
I have a question about some worm-like creatures which appear moving in my 40 gal tank. They are sitting in the gravel and moving constantly. They seem to have no colour or brownish and are very tiny. One can hardly see them at the very bottom. The tank is very clean otherwise. There was a 30% water change last week. I haven't fed live worm for a long time and I have no idea how those got in.

I wonder whether they are harmful to fish or maybe fish will eat them.

I introduced a tiny fern-like plant a couple of weeks ago. I took it from a friend's tank. I washed it well and disinfected. Then I kept it separately for a couple of days. It was supposed to be clean but who knows... I only hope those worms are not some parasites that affect fish.